When various optical devices are to be formed on a common substrate to operate with the same frequency band, it is necessary to use semiconductors of different optical characteristics, especially of different band gaps depending on the type of the optical device. Semiconductors of different band gaps are conventionally coupled optically on the same substrate by partially etching a semiconductor thin film which has been epitaxially grown, and by growing another semiconductor of a different band gap thereon. This method has been disclosed, for instance, in S. Murata, I. Mito and K. Kobayashi's paper entitled "Spectral characteristics for a 1.5 .mu.m DBR laser with frequency-tuning region", IEEE Journal of Quantum Electronics, Vol. QE-23, No. 6, June, 1987.
The prior art method is cumbersome as the steps of etching and crystal growing are repeated since the method separately grows semiconductors of different band gaps. Further, it is difficult to grow each of the semiconductor layers flatly. It is also difficult to enhance the coupling coefficient between the devices because the structures of waveguides differ from each other.
When the optical device is a semiconductor laser, it is necessary to reduce optical absorption at the laser facets in order to enhance reliability and increase output power. If optical absorption occurs at the facets, COD (catastrophic optical damage) such as melting of the facets with excessive local heat tends to take place. In order to eliminate optical absorption at the facets, the band gap at the facet should be larger than the band gap at the active region. For this purpose, there have been known the following methods; (1) the facet is etched and buried with a substance having a large band gap, or (2) the change (an increase) of a band gap caused by the disordering of a quantum well structure is used.
Method (1) is described in detail in, for instance, "Large optical cavity AlGaAs buried hetero-structure window lasers" by S. Margalit and A. Yariv; Appl. Phys. Lett. 40 (12), Jun. 15, 1982. Method (2) is described in detail in "High power (2.1 W) 10-stripe AlGaAs laser arrays with Si disordered facet windows" by R. L. Thornton, D. F. Welch, R. D. Burnham, T. L. Paoli and P. S. Cross; Appl. Phys. Lett. 49 (23), Dec. 8, 1982.
However, method (1) above is not quite satisfactory in that the manufacturing process becomes complicated as it is necessary to bury the facet portion of a semiconductor laser. Method (2) is also defective in that as the Si is diffused thermally by contacting Si on the facet, the process should be conducted separately from other semiconductor optical devices, which inevitably makes the manufacturing process complicated. Further, the place where semiconductor laser can be formed is limited on an optical IC.
This invention provides a semiconductor optical device of the structure which is easily manufactured, and which allows integration of a large number of optical devices at a high coupling coefficient, and a manufacturing method therefore. It also provides a method which can easily form a non-absorbing facet on a semiconductor laser.